A speedometer is a device that measures the instantaneous speed of a land vehicle.
Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards.
The eddy current speedometer has been used for over a century and is still in widespread use. Until the 1980s and the appearance of electronic speedometers it was the only type commonly used.
Originally patented by a German, Otto Schulze on 7 October 1902, it uses a rotating flexible cable usually driven by gearing linked to the tail shaft (output) of the vehicle's transmission. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel.
A small permanent magnet affixed to the rotating cable interacts with a small aluminum cup (called a speedcup) attached to the shaft of the pointer on the analogue instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet "drags" the cup, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.
The pointer shaft is held toward zero by a fine spring. The torque on the cup increases with the speed of rotation of the magnet (which is driven by the car's transmission.) Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. When the torque due to the eddy currents in the cup equals that provided by the spring on the pointer shaft, the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.
The return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tires.
Many modern speedometers are electronic. A rotation sensor, usually mounted on the rear of the transmission, delivers a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. The sensor is typically a toothed metal disk positioned between a coil and a magnetic field sensor. As the disk turns, the teeth pass between the two, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring. Alternatively, some manufactures rely on pulses coming from the ABS wheel sensors.
Another early form of electronic speedometer relies upon the interaction between a precision watch mechanism and a mechanical pulsator driven by the car's wheel or transmission. The watch mechanism endeavors to push the speedometer pointer toward zero, while the vehicle-driven pulsator tries to push it toward infinity. The position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms.
Changing a car's tire size can throw off a speedometer's accuracy.
Some speedometers for bicycles measure the time between each wheel revolution. The sensor is mounted on the bike at a fixed location, pulsing when the spoke-mounted magnet passes by. These digital devices can be programmed by tire size or by wheel circumference in order to make accurate distance measurements. Others are cable driven as in the automotive speedometers described above.
Most speedometers have tolerances of some 10% plus or minus due to wear on tires as it occurs. Additional sources of error are; tire diameter variations due to temperature, pressure, vehicle load, and nominal tire size.
Modern speedometers are said to be accurate within 10% but as this is legislated accuracy, this may not be entirely correct. This can make it difficult to accurately stay on the speed limits imposed; most countries allow for this known variance when using RADAR to measure speed, although levels of some 3 km/h or 3% are also used in areas of tough enforcement (e.g. Victoria, Australia). This causes many arguments due to motorists complaining that they were not doing the speed as reported. Revenue is being increasingly blamed for these stricter measures. There are strict United Nations standards in place but it seems not being enforced in some countries, as cars not adhering to United Nations standards, are imported from these, leaving this matter in limbo. Motorists are open to charges of speeding, if harsh methods 3% or 3 KPH are used using a speedometer not complying with accuracy standards.
Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the
ERRATUM: Percent error = (standard diameter – new diameter) / standard diameter * 100% This is a variation of the standard percent error formula. The one above is incorrect.
Nearly all tires now have their size shown as "T/A_W" on the side of the tire (See: Tire code), and the tire's
For example, a standard tire is "185/70R14" with diameter = 185x70/1270 + 14 = 24.20 in. Another is "195/50R15" with 195x50/1270 + 15 = 22.68 in. Replacing the first tire (and wheels) with the second (on 15" wheels), a speedometer reads 24.19/22.68 = 1.0670 times the correct speed or 6.7% too high.
ERRATUM: Using the correct formula for percent error, we conclude that the speedometer will read (24.20 – 22.68) / 24.20 * 100% = 6.28% too high e.g. at an actual speed of 60 MPH, the speedometer will indicate 60 + (60 * 0.0628) = 63.77 MPH (approx).
In many countries the legislated error in speedometer readings is ultimately governed by the United Nations Economic Commission for Europe (UNECE) Regulation 39 which covers those aspects of vehicle type approval which relate to speedometers. The main purpose of the UNECE regulations is to facilitate trade in motor vehicles by agreeing uniform type approval standards rather than requiring a vehicle model to undergo different approval processes in each country in which it is to be sold.
European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers   are similar to the UNECE regulation in that they specify that:
The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.
The UNECE regulation relaxes the requirements for vehicles mass produced following type approval. The upper limit on indicated speed is increased to 110 percent plus 6 km/h for cars, buses, trucks and similar vehicles, and 110 percent plus 8 km/h for two or three wheeled vehicles which have a maximum speed above 50 km/h (or a cylinder capacity, if powered by a heat engine, of more than 50 cc). European Union Directive 2000/7/EC, which relates to two and three wheeled vehicles, provides similar slightly relaxed limits in production.
All vehicles manufactured on or after 1 July 2007, and all models of vehicle introduced on or after 1 July 2006, must conform to UNECE Regulation 39.
The speedometers in vehicles manufactured before these dates but after 1 July 1995 (or 1 January 1995 for forward control passenger vehicles and off-road passenger vehicles) must conform to the previous Australian design rule. This specifies that they need only display the speed to an accuracy of +/- 10% at speeds above 40 km/h, and there is no specified accuracy at all for speeds below 40 km/h. There is also the added problem of cars not complying with the United Nations standards, being imported and allowed to be registered, making the situation even more complicated. This needs further investigation.
 State assemblies may also set their own requirements but (as of 2004) none specified tighter limits on the accuracy.  This has caused some controversy since it would be possible for a driver to be unaware that he is speeding should his vehicle be fitted with an under-reading speedometer. 
The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39. 
The Motor Vehicles (Approval) Regulations 2001 permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower that this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph. There is also the added problem of cars not complying with the United Nations standards, being imported and allowed to be registered, making the situation even more complicated. This needs further investigation.
As of 1997, Federal standards in the United States allowed a maximum 5% error on speedometer readings. Aftermarket modifications, such as different tire and wheel sizes or different differential gearing, can cause speedometer inaccuracy.
GPS devices are capable of showing speed readings based on how far the receiver has moved since the last measurement (a second ago). As the GPS is an independent* system, its speed calculations are not subject to the same sources of error as the vehicle's speedometer. Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower. The GPS software may also use a moving average calculation to reduce error.
As mentioned in the satnav article, GPS data has been used to overturn a speeding ticket; the GPS logs showed the defendant traveling below the speed limit when they were ticketed. That the data came from a GPS device was likely less important than the fact that it was logged; logs from the vehicle's speedometer could likely have been used instead, had they existed.
* some satnav devices may also use data from the car's systems to improve accuracy